1. Field of the Invention
The present invention relates to an improved explosive connecting cord for use in transmitting a detonation wave to an explosive charge, and more particularly to an explosive connecting cord of the type known as "low-energy detonating cord". The invention relates also to a method and apparatus for manufacturing detonating cord.
2. Description of the Prior Art
The hazards associated with the use of electrical initiation systems for detonating explosive charges in mining operations, i.e., the hazards of premature initiation by stray or extraneous electricity from such sources as lightning, static, galvanic action, stray currents, radio transmitters, and transmission lines, are well-recognized. For this reason, non-electric initiation through the use of a suitable detonating fuse or cord has been looked upon as a widely respected alternative. A typical high-energy detonating cord has a uniform detonation velocity of about 6000 meters per second and comprises a core of 30-50 grains per foot (6 to 10 grams per meter) of pentaerythritol tetranitrate (PETN) covered with various combinations of materials, such as textiles, waterproofing materials, plastics, etc. However, the magnitude of the noise produced when a cord having such PETN core loadings is detonated on the surface of the earth, as in trunklines, often is unacceptable in blasting operations in developed areas. Also, the brisance (shattering power) of such a cord may be sufficiently high that the detonation impulse can be transmitted laterally to an adjacent section of the cord or to a mass of explosive which, for example, the cord contacts along its length. In the latter situation, the cord cannot be used to initiate an explosive charge in a borehole at the bottom (the "bottomhole priming" technique), as is sometimes desired.
Low-energy detonating cord (LEDC) was developed to overcome the problems of noise and high brisance associated with the above-described 30-50 grains per foot cord. LEDC has an explosive core loading of only about 0.1 to 10 grains per linear foot (0.02 to 2 grams per meter) of cord length, and often only about 2 grains per foot (0.4 gram per meter). This cord is characterized by low brisance and the production of little noise, and therefore can be used as a trunkline in cases where noise has to be kept to a minimum, and as a downline for the bottomhole priming of an explosive charge.
U.S. Pat. No. 2,982,210 describes a low-energy detonating cord comprised of a continuous core of a granular cap-sensitive high explosive such as PETN of such diameter as to contain from 0.1 to 2 grains per foot (0.02 to 0.4 g/m) of explosive, encased in a metal sheath, which may be covered with a fabric countering or a coating of plastic. The metal sheath is reported to be essential for the propagation of detonation in explosive cores of such low loadings.
Because LEDC having a metal sheath is not amenable to continuous manufacture in unlimited length, and because this cord is electrically conductive along its length owing to the conductivity of the metal sheath, attempts have been made to eliminate the metal sheath by resorting to other expedients to overcome the effect of its absence. Such attempts have not always met with complete success especially with core loadings of about 2 grains per foot (0.4 g/m) or less. For example, it is stated in U.S. Pat. No. 3,125,024 that a uniform detonation velocity can be obtained even without a metal sheath with a granular PETN core in loadings of 1.5 to 10 grains per foot (0.32 to 2 grams per meter) of length provided that the specific surface of the PETN is from about 900 to 3400 square centimeters per gram and the granular core is confined within a woven textile sheath surrounded by a protective and reinforcing covering, i.e., a thermoplastic layer or a series of waterproofing and reinforcing materials including a second textile sheath. However, woven or wound sheathing is relatively expensive to apply, both in terms of the type of equipment required and limitations thereby imposed on cord production rates. Furthermore, even with the high PETN specific surface and the confinement afforded by the woven textile sheath and thermoplastic covering, reliable, high-velocity detonation is not achieved when the PETN core loading is at the lower end of the LEDC range.
British Pat. No. 815,534 and U.S. Pat. No. 3,311,056 describe low-energy detonating cords having an explosive core confined within a polymeric sheath. The British patent describes a cord having a granular core of finely divided explosive in loadings from 2 to 15 grains per foot (0.4 to 3 g/m) confined in a flexible sheath of a thermoplastic polymer, which may be wrapped in woven fabric and wire for strength and abrasion resistance. The detonating cord described in U.S. Pat. No. 3,311,056 is a non-rupturing type of cord by virtue of a thick expandable sheath of elastomeric polyurethane which surrounds the explosive core, the ratio of the amount of explosive in grains per foot to sheath thickness in inches to prevent rupturing being less than 130/1, and preferably from about 10/1 to 100/1 (ratio of the amount of explosive in grams per meter to sheath thickness in centimeters less than 11/1, and preferably from about 0.8/1 to 8/1). Explosive core loadings of 1 to 400, preferably 2 to 100, grains per foot (0.2 to 80, preferably 0.4 to 20, grams per meter) are described, and thus the cord encompasses high-energy as well as low-energy detonating cords. The 2-20 grains per foot (0.4-4 grams per meter) cord claimed has a PETN core confined in a lead sheath. Moreover, although explosive cores made of self-supporting compositions of the type used in sheet explosives, e.g., those shown in U.S. Pat. Nos. 2,992,087 and 2,999,743, are disclosed, the low-energy detonating cores having loadings of 5 and 10 grains per foot (1 and 2 grams per meter) have granular explosive cores, confining lead sheaths, and low ratios of explosive loading to polyurethane sheath thickness (48 and 20 grains per foot per inch, 4 and 1.7 grams per meter per centimeter, of sheath thickness).
U.S. Pat. No. 3,384,688 describes the preparation of a textile-sheathed cord having enhanced sensitivity to side initiation and the ability to propagate detonation at lower loading densities by the use of a special finely divided granular PETN core in a loading of 10 grains per linear foot (2 g/m). U.S. Pat. No. 3,382,802 prescribes a maximum particle size of 100 microns, with at least half the particles smaller than 50 microns, for a core of granular primary explosive in low loading, e.g., 5-10 grains per foot (1-2 grams per meter), encased in a sheath of spiral-wound thread-like elements made of metal or thermoplastic, spiral-wound fibrous sheaths, and a thermoplastic outer shell.
As can be seen from the above-discussed patents, heretofore granular explosive cores have been used in detonating cords having core loadings of 10 grains per foot (2 grams per meter) or less. Moreover, metal or heavy textile sheathing generally has been indicated, especially when the loading drops to below 2 grains per foot (0.4 grams per meter). Self-supporting explosive compositions in which a crystalline high explosive compound is admixed with a binding agent can be extruded rapidly in the form of cords and would enable higher cord production rates to be attained as contrasted to production rates attainable with cords having granular cores. Also, bonded explosive compositions have high density and can detonate at a higher velocity for a given diameter when contrasted to lower-density explosives. However, since the common bonded explosive compositions contain less-sensitive materials, such compositions are less sensitive to initiation than totally explosive granular compositions and would not be expected to detonate under all of the same conditions as such granular compositions. Thus, while U.S. Pat. No. 3,311,056 describes certain detonating cords having bonded explosive cores, the low-loading cores therein are granular PETN and lead azide/aluminum, and even these are lead-jacketed. Also, it is known that the cord diameter and explosive loading have to be sufficiently large if self-supporting sheet explosive compositions are to propagate a detonation at uniformly high velocity. The aforementioned U.S. Pat. No. 2,992,087 discloses that a cord made by extruding a nitrocellulose-based PETN sheet explosive to a PETN loading of 20 grains per foot (4 grams per meter) detonates at a velocity of greater than 6400 meters per second; and the aforementioned U.S. Pat. No. 3,311,056 discloses bonded-explosive cores in PETN loadings of 17.5 and 20.0 grains per foot (3.7 and 4 grams per meter). However, cords having bonded-explosive cores in explosive loadings of 10 grains per foot (2 grams per meter) or less have been avoided despite the fact that such loadings have been found operable with granular PETN explosives. U.S. Pat. Nos. 3,338,764, 3,401,215, 3,407,731, and 3,428,502 describe the preparation of detonating cord having an explosive loading of 50 to 200 grains per foot (10 to 40 grams per meter) by the extrusion of a flexible elastomer-bonded explosive composition, preferably around an axially positioned reinforcing yarn or thread. The wrapping of reinforcing yarns or threads around the extruded cord, e.g., as in a braided structure, and the bonding of the yarns to the cord with a latex or liquid polymer is reported to be less desirable than an internally placed reinforcing means.
In the art of manufacturing detonating cord, threads have been used also for the purpose of facilitating the sheathing of powdered explosive cores. For example, U.S. Pat. No. 3,683,742 describes circularly guiding one or more roughened threads through a funnel which feeds dust-like explosive into a sheath continuously manufactured at the lower end of the funnel, the thread(s) being deflected from the funnel's vertical axis and introduced into the sheath together with the explosive. The thread(s) entrain the dust-like explosive and conduct it into the sheath, whereby a granular explosive core is formed around internal thread(s).
British Pat. No. 1,416,128 and Belgian Pat. No. 815,257 describe enclosing a column of dry, pulverulent explosive within a boundary of joined-together axial threads, and drawing the column/thread assembly through a compressing die under a tension exerted on the threads so as to form the core of a detonating fuse. The thus-formed core, in which the threads enwrap and form a sleeve around the explosive, is shown enwrapped with a reinforcing layer of wound textile material, which is coated with plastic for waterproofing.
U.S. Pat. No. 2,687,553 describes the use of longitudinal threads in cord manufacture for the purpose of reinforcing a thermoplastic coating to overcome the latter's elasticity. The resulting cord has an explosive core enclosed in a sheath of thermoplastic material in which strong threads are embedded in a longitudinal direction. The entire periphery of the explosive core is in direct contact with the thermoplastic sheath, and the threads are surrounded by the thermoplastic.